74 Mr. H. Collen on the Application of Photography. 



meter, the barometer, and the thermometer, and a series of 

 experimental observations permanently registered at Kew. 



The accompanying figure (Plate III. fig. 1) is part of a 

 day's registration of the effect of atmospheric electricity on 

 Volta's electrometer ; the gradual decline of daylight is shown, 

 and also the continuation of the registration, by artificial 

 light; without the use of the latter, it is obvious that the ap- 

 plication of photography to these purposes would be very in- 

 complete, if not wholly useless ; and it may perhaps, in some 

 cases, be advisable to make its use constant. 



The various intensities of light from a clouded sky frequently 

 give rise (of course) to variations in depth of tint on the paper, 

 which thus becomes an approximation to Sir John Herschel's 

 actinograph ; and-it may be here worth while to remark, that 

 sometimes, when with such a sky these intensities of action on 

 the paper are augmented, the electricity of serene weather ma- 

 nifests a tendency to increase also; this fact may be compared 

 with the almost invariable tendency of the sun's light and heat, 

 in a clear sky, to diminish the tension of those electrometers 

 which receive their charges by absorption. 



The calotype process is that which is used, being, of all 

 those upon paper, the most sensitive, which quality is highly 

 essential during the use of artificial light; it is very advan- 

 tageously employed for these purposes, instead of the Da- 

 guerreotype, on account of its cheapness, and also on account 

 of the facility with which representations can be obtained of 

 any required length. 



In the apparatus at present constructed, the paper is moved 

 by a clock at the rate of one inch per hour, and is cut into 

 pieces nine inches long; but for constant use they should be 

 twelve inches long, so that by the introduction of two pieces 

 during twenty-four hours, a continuous register of the effects 

 would be preserved without further attention than the appli- 

 cation of the artificial light (if not used constantly) at the de- 

 cline of daylight; at present an Argand lamp is used, which, 

 of course, requires some attention, but where available, a 

 common gas-light would be greatly preferable; this however 

 is not the case at the Kew Observatory, and for this reason 

 only, the experiments have not been continued during the 

 night. 



The construction of the apparatus is very simple, although 

 many tedious experiments have been made to produce the re- 

 sult; it consists essentially of the following arrangement: — 

 The instrument to be registered is placed so as to be between 

 the light and a lens of considerable aperture, with very short 

 focus, and flat field of sufficient extent for the purpose ; and 



